MECHANICAL, THERMAL AND MORPHOLOGICAL...
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MECHANICAL, THERMAL AND MORPHOLOGICAL PROPERTIES OF MONTMORILLONITE FILLED LINEAR LOW DENSITY POLYETHYLENE-TOUGHENED POLYLACTIC ACID NANOCOMPOSITES HARINTHARAVIMAL BALAKRISHNAN UNIVERSITI TEKNOLOGI MALAYSIA
Transcript of MECHANICAL, THERMAL AND MORPHOLOGICAL...
MECHANICAL, THERMAL AND MORPHOLOGICAL PROPERTIES OF
MONTMORILLONITE FILLED LINEAR LOW DENSITY
POLYETHYLENE-TOUGHENED POLYLACTIC ACID NANOCOMPOSITES
HARINTHARAVIMAL BALAKRISHNAN
UNIVERSITI TEKNOLOGI MALAYSIA
MECHANICAL, THERMAL AND MORPHOLOGICAL PROPERTIES OF
MONTMORILLONITE FILLED LINEAR LOW DENSITY
POLYETHYLENE-TOUGHENED POLYLACTIC ACID NANOCOMPOSITES
HARINTHARAVIMAL BALAKRISHNAN
A thesis submitted in fulfillment of the
requirements for the award of the degree of
Master of Engineering (Polymer)
Faculty of Chemical and Natural Resources Engineering
Universiti Teknologi Malaysia
JANUARY 2010
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‘To mankind, may knowledge and wisdom endeavor us with capabilities for our
challenging future ahead’
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ACKNOWLEDGEMENTS
First and foremost, I would like to express my heartfelt gratitude to my
supervisor, Professor Dr. Azman Hassan, for his ever-lasting enthusiasm,
encouragement, excellent advice and great concern to my work. A sincere thanks is
accorded to my co-supervisor, Assoc. Prof. Dr. Mat Uzir Wahit for his guidance,
suggestions, motivation and encouraging advices.
I also wish to express my appreciation to all the lecturers in the Department of
Polymer Engineering, the Quality Control staffs in Poly-Star Compounds Sdn. Bhd.,
Malaysian Institute of Nuclear Technologies (MINT) and Malaysian Rubber Board
(LGM) for their help and support in my research. A heartfelt line of appreciation to all
the technicians and my labmates of Enhanced Polymers Research Group (EnPRO) for
creating a friendly and enjoyable working environment.
Besides, I would like to acknowledge IRPA grant 79162 and the Ministry of
Science, Technology and Innovation (MOSTI), Malaysia for generous financial funding
and Research Student Grant throughout my studies.
Last but not least, I wish to extend my deep appreciation to my beloved father,
Mr. Balakrishnan, my brothers and Shree, who have always encouraged and provided
me with moral support in completing this thesis.
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ABSTRACT
Linear low density polyethylene (LLDPE) toughened polylactic acid (PLA)
nanocomposites containing organophilic modified montmorillonite (MMT) were
prepared by melt extrusion using a counter-rotating twin-screw extruder followed by
injection molding in order to examine the mechanical, morphological and thermal
properties of the nanocomposites. The mechanical properties of PLA/LLDPE
nanocomposites were studied through tensile, flexural and impact tests. Scanning
electron microscopy (SEM) was used to investigate the phase morphology and LLDPE
particle’s size in PLA/LLDPE blends and nanocomposites. X-ray diffraction (XRD) was
employed to characterize the formation of nanocomposites while the thermal properties
were determined using thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC). The dynamic mechanical properties were examined via dynamic
mechanical analysis (DMA) while moisture permeability properties of the PLA/LLDPE
nanocomposites were assessed through water absorption and hygrothermal aging.
Subsequently, for PLA/LLPDE blends, the loadings of LLPDE were varied from 5-15
wt% and PLA/LLDPE nanocomposites with 2 phr and 4 phr loadings of MMT were
prepared only for the optimum formulation (10 wt% of LLDPE). The results showed
that the blending of LLDPE significantly increased the toughness but at the expense of
stiffness and strength. Conversely, the incorporation of the MMT increased the stiffness,
while the toughness and strength decreased. The PLA/LLDPE nanocomposites
containing 2 phr of MMT and 10 wt% of LLDPE had the best balance of stiffness,
strength and toughness. The impact strength results also proved that PLA
nanocomposites were successfully toughened with LLDPE. XRD established that MMT
were well dispersed and preferentially embedded in the PLA phase. SEM revealed that
blend ratio and the presence of MMT were found to influence the morphology (e.g.
LLDPE particle size and distribution) of the system. Finer particles’ size and better
distribution of LLDPE has been observed in higher MMT loadings in the system. The
SEM micrographs also revealed that increasing content of LLDPE has increased the
particle size of LLDPE in PLA. DMA analysis discovered that the storage modulus at
30ºC increased with the presence of MMT for PLA nanocomposites. The DSC results
showed that the crystallization temperature (Tc) dropped gradually with increasing
content of MMT for both PLA and PLA/LLDPE nanocomposites while the glass
transition (Tg) and melting temperature (Tm) remained unchanged. TGA also exhibited
an increase in T10% decomposition temperature for PLA and PLA/LLDPE
nanocomposites. Water absorption curves obeyed the Fick’s law with rapid moisture
absorption to maximum saturation level (Mm) and the value of Mm of PLA increased
with addition of LLDPE and 2 phr of MMT. Hygrothermal aging revealed that the Mm
increased significantly at elevated temperatures (60ºC and 90ºC) and addition of LLDPE
and MMT improved the hygrothermal stability of PLA.
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ABSTRAK
Nanokomposit asid polilaktik (PLA) yang mengandungi montmorillonite (MMT)
yang diubahsuai secara organofilik, diliat dengan polietilena berantai lurus
berketumpatan rendah (LLDPE), telah disediakan melalui penyemperitan leburan
menggunakan penyemperit skru berkembar berlawanan arah diikuti proses acuan
suntikan untuk mengkaji sifat mekanikal, morfologi dan terma nanokomposit tersebut.
Sifat mekanikal nanokomposit PLA/LLDPE dikaji berdasarkan ujian tegangan, lenturan
dan hentaman. Mikroskop Imbasan Elektron (SEM) digunakan untuk mengkaji
morfologi dan saiz partikel LLDPE dalam adunan PLA/LLDPE dan nanokompositnya.
Pembelauan sinar-X (XRD) digunakan untuk pencirian pembentukan nanokomposit
sementara sifat termal ditentukan dengan menggunakan analisis termogravimetrik
(TGA) dan kalorimeter pembezaan imbasan (DSC). Sifat dinamik diperiksa dengan
menggunakan penganalisis terma dinamik (DMA) sementara sifat kebolehtelapan
lembapan nanokomposit PLA/LLDPE dikaji melalui penyerapan air dan penuaan
higroterma. Seterusnya, untuk adunan PLA/LLDPE, kandungan LLDPE diubah dari 5
hingga 15 wt% dan nanokomposit PLA/LLDPE dengan kandungan MMT 2 phr dan 4
phr disediakan hanya untuk formulasi yang optimum (10 wt% LLDPE). Keputusan
menunjukkan adunan LLDPE jelas meningkatkan keliatan tetapi menyebabkan
penurunan kekakuan dan kekuatan bahan. Sebaliknya campuran dengan MMT
meningkatkan kekakuan namun keliatan dan kekuatan menurun. Nanokomposit
PLA/LLDPE yang mengandungi 2 phr MMT dan 10 wt% LLDPE adalah formulasi
yang terbaik dengan keseimbangan kekakuan, kekuatan dan keliatan. Keputusan
kekuatan hentaman membuktikan bahawa nanokomposit PLA berjaya diliatkan dengan
LLDPE. XRD menunjukkan MMT diselerakkan dengan baik dan berada di dalam
matrik PLA. SEM mendedahkan nisbah adunan dan kehadiran MMT didapati
mempengaruhi morfologi sistem (saiz partikel dan keserakan LLDPE). Partikel saiz
LLDPE yang kecil dan keserakan yang baik dapat dilihat dengan kandungan MMT yang
banyak. Mikrograf SEM juga mendedahkan peningkatan kandungan LLDPE telah
meningkatkan saiz partikel LLDPE dalam PLA. Analisis DMA mendapati bahawa
modulus simpanan pada 30 �C meningkat dengan kehadiran MMT dalam nanokomposit
PLA. Keputusan DSC menunjukkan suhu penghabluran (Tc) turun beransur-ansur
dengan peningkatan kandungan MMT dalam kedua-dua PLA dan nanokomposit
PLA/LLDPE, sementara suhu peralihan kaca (Tg) dan takat lebur (Tm) kekal tidak
berubah. TGA juga menunjukkan suhu penguraian T10% meningkat untuk PLA dan
nanokomposit PLA/LLLDPE. Lengkungan penyerapan air mematuhi hukum Fick
dengan penyerapan lembapan pantas ke tahap ketepuan maksimum (Mm) dan nilai Mm
untuk PLA meningkat dengan penambahan LLDPE dan MMT. Penuaan higroterma
menunjukkan bahawa Mm meningkat pada suhu (60�C dan 90�C) dan penambahan
LLDPE dan MMT meningkatkan kestabilan higroterma PLA.
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TABLE OF CONTENT
CHAPTER TITLE PAGE
TITLE i
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENTS iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLES xi
LIST OF FIGURES xii
LIST OF ABBREVIATIONS AND SYMBOLS xv
LIST OF APPENDICES xvii
1 INTRODUCTION 1
1.1 Background 1
1.2 Objectives of the study 4
1.3 Scope of Research 5
1.4 Importance of Research 6
2 LITERATURE REVIEW 7
2.1 Biopolymers 7
2.2 Polylactic acid (PLA) 7
2.3 Plasticized PLA 12
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2.4 Nanocomposites 13
2.5 Montmorillonite (MMT) 14
2.5.1 Morphology and Structure 15
2.5.2 Clay characteristics 16
2.5.3 Clay Surface Modification 17
2.5.4 Clay Distribution 18
2.6 Nanocomposites preparation methods 19
2.7 Polylactic acid nanocomposites 23
2.8 Plasticized polylactic acid nanocomposite 30
2.9 Toughened polymers 32
2.10 Toughened polylactic acid 34
2.11 Toughened polylactic acid nanocomposites 36
2.12 Characterization of Nanocomposites 37
2.12.1 X-Ray Diffraction (XRD) 37
2.12.2 Transmission Electron Microscopy (TEM) 38
2.13 Water absorption 41
2.13.1 Mechanism of moisture penetration 41
2.13.2 Diffusion 41
2.14 Hygrothermal aging 43
2.14.1 Degradation and Hydrolysis 44
3 MATERIALS AND METHODS 47
3.1 Materials 47
3.1.1 Thermoplastics 47
3.1.2 Montmorillonite (MMT) 48
3.1.3 Composition and Designation of Materials 48
3.2 Sample Preparation 49
3.2.1 Premixing 49
3.2.2 Extrusion 49
3.2.3 Injection Moulding 50
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3.3 Materials Properties Characterization 50
3.3.1 Mechanical Testing 50
3.3.1.1 Tensile 50
3.3.1.2 Flexural 50
3.3.1.3 Notched Izod Impact 51
3.3.2 Morphological Study 51
3.3.2.1 X-ray Diffraction (XRD) 51
3.3.2.2 Scanning Electron Microscopy (SEM) 51
3.3.2.3 Transmission Electron Microscopy (TEM) 52
3.3.3 Thermal Analysis 52
3.3.3.1 Dynamic Mechanical Analysis (DMA) 53
3.3.3.2 Differential Scanning Calorimetry (DSC) 53
3.3.3.3 Thermogravimetry Analysis (TGA) 53
3.3.4 Water Absorption 54
3.3.5 Hygothermal aging 55
4 RESULTS AND DISCUSSION 56
4.1 Effect of LLDPE contents on PLA/LLDPE blends 56
4.1.1 Mechanical Properties 56
4.1.1.1 Izod Impact Properties 56
4.1.1.2 Tensile properties 58
4.1.1.3 Elongation at break 60
4.1.1.4 Flexural properties 61
4.1.1.5 Overall mechanical properties 62
4.1.2 Morphology studies (Scanning electron microscopy) 64
4.1.3 Thermal properties 67
4.1.3.1 Differential scanning calorimetry (DSC) 67
4.1.3.2 Thermogravimetric analysis (TGA) 68
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4.2 Effect of MMT on PLA/MMT and PLA/LLDPE/MMT
nanocomposites 71
4.2.1 Mechanical properties 71
4.2.1.1 Overall mechanical properties 76
4.2.2 Morphological properties 77
4.2.2.1 X-Ray Diffraction (XRD) 77
4.2.2.2 Transmission electron microscope (TEM) 82
4.2.2.3 Scanning Electron Microscopy (SEM) 87
4.2.3 Thermal properties 90
4.2.3.1 Differential scanning calorimetry (DSC) 90
4.2.3.2 Thermogravimetry analysis (TGA) 92
4.2.3.3 Dynamic mechanical analysis (DMA) 96
4.3 Water absorption 102
4.3.1 Kinetics of water absorption 102
4.4 Hygrothermal aging 105
5 CONCLUSION AND RECOMMENDATIONS 112
5.1 Conclusion 112
5.2 Recommendations for future works 114
REFERENCES 116
Appendices A-E 125
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LIST OF TABLES
TABLE NO. TITLE PAGE
2.1 Mechanical properties of PLA (Material datasheet by Biomer for L9000) 10
3.1 Blend composition of PLA/LLDPE 48
3.2 Blend composition of PLA with different MMT loadings 48
3.3 Blend composition of PLA/LLDPE (90/10) with different content of MMT 59
4.1 Thermal characteristics of PLA and PLA/LLDPE blends 68
4.2 TGA results of PLA and PLA/LLDPE blends 70
4.3 The 2� angle and d-spacing of MMT for PLA/MMT and
PLA/LLDPE/MMT nanocomposites 79
4.4 DSC results for PLA/MMT and PLA/LLDPE/MMT nanocomposites 92
4.5 TGA results for PLA/MMT and PLA/LLDPE/MMT nanocomposites 96
4.6 Equilibrium water content, Mm and diffusivity, D of
PLA/MMT and PLA/LLDPE/MMT nanocomposites 103
4.7 Effect of immersion temperature on equilibrium moisture content
(Mm) of PLA/MMT and PLA/LLDPE/MMT nanocomposites 107
4.8 Calculated weight loss of PLA in percentage (%) in
PLA/MMT and PLA/LLDPE/MMT nanocomposites at 30 days 111
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LIST OF FIGURES
FIGURE NO. TITLE PAGE
2.1 Lifecycle of polylactic acid (PLA) 9
2.2 Mechanical properties of PLA and other commodity plastics:
(a) Young’s Modulus (b) Tensile strength (c) Elongation at break 11
2.3 Structure of 2:1 phyllosilicates (Ray and Okomoto 2003) 15
2.4 Surface modification of montmorilonite (Lim, 2006) 18
2.5 Three possible structures of polymer-layered silicate composites 19
2.6 Schematic representation of polymer nanocomposite obtained
by in-situ polymerization 20
2.7 Schematic representation of polymer-layered silicate nanocomposite
obtained by intercalation of polymer from solution 21
2.8 Schematic representation of polymer-layered silicate
nanocomposite obtained direct melt intercalation 22
2.9 Typical XRD patterns for organically modified layered
silicates (OMLS) and various types of nanocomposites 39
2.10 TEM images of three different types of nanocomposites 40
2.11 Reaction pathway of hydrolysis in PLA 45
4.1 Effect of LLDPE content on the Izod Impact strength of
PLA/LLDPE blends 57
4.2 Effect of LLDPE content on the Young’s modulus and
tensile strength of PLA/LLDPE blends 59
4.3 Typical stress-strain curves for PLA/LLDPE blends 59
4.4 Effect of LLDPE content on Elongation of break of PLA/LLDPE blends 60
4.5 Effect of LLDPE content on flexural strength and modulus
of PLA/LLDPE blends 62
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4.6 Determination of balance properties based on flexural modulus
and impact strength of PLA/LLDPE blends 63
4.7 Representative SEM images of impact fractured surfaces:
(a) PLA, (b) P95/L5, (c) P90/L10, (d) P85/L15 66
4.8 Correlation between impact strength and LLDPE particles size 66
4.9 Thermogravimetric analysis of PLA and PLA/LLDPE with
different LLDPE contents 70
4.10 The effect of MMT content on Young’s modulus and
tensile strength of PLA/MMT and PLA/LLDPE/MMT nanocomposites 73
4.11 The effect of MMT content on the flexural modulus and flexural
strength of PLA/MMT and PLA/LLDPE/MMT nanocomposites 74
4.12 The effect of MMT content on the impact strength and elongation
at break of PLA/MMT and PLA/LLDPE/MMT nanocomposites 76
4.13 Determination of balanced properties based on flexural modulus and
impact strength of PLA/MMT and PLA/LLDPE/MMT nanocomposites 77
4.14 XRD of (a) neat PLA, (b) PLA/M2, (c) PLA/M4 and (d) neat MMT 79
4.15 XRD of (a) PLA/LLDPE, (b) PLA/LLDPE/M2, (c) PLA/LLDPE/M4
and (d) neat MMT 80
4.16 XRD of (a) neat PLA, (b) PLA/M2, (c) PLA/M4,
(d) PLA/LLDPE, (e) PLA/LLDPE/M2, (f) PLA/LLDPE/M4 82
4.17 TEM micrographs of PLA and PLA/MMT nanocomposites 84
4.18 TEM micrographs of PLA/LLDPE and PLA/LLDPE/MMT
nanocomposites 86
4.19 Representative SEM images of Izod impact fractured surfaces:
(a) PLA/LLDPE, (b) PLA/LLDPE/M2, (c) PLA/LLDPE/M4 89
4.20 TGA curves of PLA and PLA/MMT nanocomposites 93
4.21 TGA curves for PLA/LLDPE and PLA/LLDPE/MMT nanocomposites 94
4.22 Storage modulus of PLA and PLA/MMT nanocomposites 98
4.23 Storage modulus of PLA/LLDPE and PLA/LLDPE/MMT nanocomposites 100
4.24 Loss modulus of PLA and PLA/MMT nanocomposites 101
4.25 Loss modulus of PLA/LLDPE and PLA/LLDPE/MMT nanocomposites 102
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4.26 Water absorption curves of PLA/MMT and PLA/LLDPE/MMT
nanocomposites 105
4.27 Water absorption curves for PLA/MMT and PLA/LLDPE/MMT
nanocomposites at 60ºC 106
4.28 Hygrothermal aging of PLA/MMT and PLA/LLDPE/MMT
nanocomposites at 60ºC 107
4.29 Hygrothermal aging of PLA/MMT and PLA/LLDPE/MMT
nanocomposites at 90ºC 109
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LIST OF ABBREVIATIONS AND SYMBOLS
ASTM American Society for Testing and Materials
CEC Cation Exchange Capacity
D Diffusivity
DMA Dynamic Mechanical Analysis
DSC Differential Scanning Calorimeter
d Spacing between diffractional lattice plane (interspacing)
E’ Storage Modulus
E” Loss Modulus
ENR Epoxidized Natural Rubber
HDPE High Density Polyethylene
Hz Hertz
LDPE Low Density Polyethylene
LLDPE Linear Low Density Polyethylene
MFR Melt flow rate
MMT Montmorillonite
Mm Moisture absorption at maximum
Mt Percentage weight at time, t
NBR Acrylonitrile butadiene rubber
NR Natural rubber
PA Polyamide
PA6 Polyamide 6
PE Polyethylene
PEG Polyethylene glycol
PET Polyethyelene terephtalate
PLA Polylactic acid
PS Polystyrene
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PVC Polyvinyl chloride
SBR Styrene butadiene rubber
SEM Scanning electron microscopy
tan � Tangent delta
TEM Transmission electron microscopy
Tg Glass transition temperature
Tm Melting temperature
Tc Crystallization temperature
TGA Thermogravimetric analysis
Wd Weight of dry sample
Ww Weight of samples after exposure to distilled water
XRD X-ray diffraction
Xc Degree of crystallinity
� Diffraction angle
� Wave length
�Hm Heat of fusion for sample
�Hmideal Heat of fusion for 100% crystalline
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LIST OF APPENDICES
APPENDIX TITLE PAGE
A. Paper 1 (Abstract) Journal of Elastomers and Plastics 126
B. Paper 2 (Abstract) Materials and Design 127
C. Paper 3 (Abstract) 128
D. Conference proceeding (RAFSS 2008), Malaysia. 129
E. Conference Proceeding (NSPM 2008), Malaysia. 130
CHAPTER 1
INTRODUCTION
1.1 Background
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Biopolymers have been studied extensively due to environmentally aware
consumers, increased price of crude oil and global warming. These polymers are derived
from naturally occurring polymers that are found in all living organisms. Biopolymers
which sourced from renewable resources will reduce our dependence on petroleum
(Peterson and Oksman, 2006). Today, biopolymers are used in a variety of applications
such as therapeutic aids, medicines, coatings, food product and packaging materials.
Polylactic acid (PLA) has caught the attention of polymer scientist recently as a
potential biopolymer to substitute the conventional petroleum based plastics. Apart from
being in the category of biodegradable polymer, PLA has wide applications in
biomedical field due to its biocompatibility characteristics. Recent studies on PLA had
concluded that the biopolymer has good mechanical properties, thermal plasticity and
biocompatibility, and is readily fabricated, thus being a promising polymer for various
end-use applications (Ray et al., 2003). However, PLA, similar to polystyrene, is a
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comparatively brittle and stiff polymer with low deformation at break and low impact
strength (Jacobsen and Fritz, 1999).
One main task will be to modify these properties in such a way that PLA is able
to compete with other more flexible commodity polymers such as polyethylene (PE),
polypropylene (PP), polyethylene terepthalate (PET) or polyvinyl chloride (PVC).
Previous researchers (Jacobsen and Fritz, 1999; Baiardo et al., 2003; Kulinski and
Piorkowska, 2005) had used plasticizers to enhance PLA’s elongation at break and
reduce its brittleness. Jacobsen et al. (1999) had studied the introduction of plasticizers
such as polyethylene glycol (PEG), glucosemonoesters and partial fatty acid esters into
PLA. They showed that the impact strength of PLA had increased significantly from 31
to 80 J/m with the addition of 10% PEG, leading to a toughened PLA.
The recent achievements in nanocomposite technology has fueled the need for
new knowledge and findings resulting development of respective polymer
nanocomposites; polyamide (Kelnar et al., 2005), polypropylene (Lim et al., 2006),
polycaprolactone (Lepoittevin et al., 2002), polystyrene (Fu and Qutubuddin, 2000) and
others. In recent years, the field of PLA nanocomposites (Ray et al., 2003; Lee et al.,
2003; Petersson and Oksman, 2006) based on layered silicates, such as MMT, has
steadily increasing interest from scientist and industrialist. The nanoscale distribution of
such high aspect ratio fillers brings up some large improvements to the polymer matrix
in terms of mechanical, fire retardant, rheological, gas barrier and optical properties,
especially at low clay content (as small as 1wt%) in comparison with conventional
microcomposites (>30 wt% of microfiller). In order to reach this nanoscale distribution,
the naturally hydrophilic clay filler has to be organically modified to be compatible with
the organic polymer matrix.
Interestingly, the distribution of nanoclay is proven to be well dispersed in PLA
without introduction of compatibilizing agents due to the interaction of hydrogen
bonding between ammonium group in the organic “surfactant” of the MMT with the
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carbonyl group of PLA chain segments contributes to this process (Pluta, 2006). There
are also strong interactions between the PLA hydroxyl end groups and the MMT platelet
surfaces or the ammonium group of the ammonium surfactant in the organically
modified MMT reported in previous studies (Jiang et al., 2007). Regardless of the
improvements achieved in the development of PLA nanocomposites, the polymer’s
brittleness had become more inherent. This had limited its applications.
Similar brittleness problems had been solved before by researches with the
approach of introduction of tough materials into a brittle nanocomposite system. Ahn et
al. (2006) had examined the rubber toughening of nylon 6 nanocomposites in terms of
impact strength, ductile–brittle transition temperature, and tensile properties. Lim et al.
(2006) had successfully developed the poly(ethylene-co-octene) toughened
polypropylene nanocomposites and studied its morphology, thermal and mechanical
behaviour. Only several studies had been conducted recently in the development of
plasticized PLA nanocomposites (Pluta, 2004; Paul et al., 2003; Thellen et al., 2005) but
the mechanical properties of these nanocomposites were not studied in detail. Thellen et
al. (2005) had investigated the influence of MMT layered silicate on plasticized PLA
blown films and concluded that the plasticized PLA/MMT nanocomposites did not see
highly significant enhancements with addition of MMT but the toughness is at least
maintained in the nanocomposites unlike other filled polymeric systems. They found
that the plasticization effect reduced the brittleness of the nanocomposites and the
breakthrough will widen PLA’s applications.
Although the addition of plasticizers such as PEG will overcome the brittleness,
it comes with a sacrifice of stiffness of the material which is also important for structural
applications (Baiardo et al., 2003). Jacobsen et al. (1999) had proved that the addition of
a plasticizer (PEG) leads to a decrease of the elasticity modulus and the addition of 2.5
wt% already lowers the modulus by 10% to 15% and this result was nearly
independently of the type of plasticizer used. Thus, a new toughener was needed to
overcome the brittleness nevertheless significantly reducing the stiffness. The
introduction of LLDPE as a toughener for PLA indeed showed a remarkable
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enhancement in terms of toughness of the material. The results showed a significant
improvement in impact strength with considerable reduction in tensile and flexural
strength and modulus. It was also concluded that the partial miscibility of LLDPE in
PLA enabled it to perform as an impact modifier for PLA (Anderson et al., 2003).
Although it seems that the use of LLDPE as an impact modifier for PLA may defeat the
purpose of developing a ‘green’ polymer, it shall be noted that the main aim of this
research is for utilization of PLA in structural applications such as furniture and
automobile parts.
1.2 Objectives of the study
One of the most important aspects of materials development in thermoplastics
engineering is to achieve a good combination of mechanical properties and
processability at a moderate cost. As far as mechanical properties are concerned, the
main target is to strike a balance of stiffness, strength and toughness. To the best of our
knowledge, no study on LLDPE toughened PLA nanocomposites have been reported
yet. Thus, the aim of the research is to develop an environmentally friendly polymer
nanocomposite with enhanced toughness namely LLDPE-toughened PLA/MMT
nanocomposite.
The main objective can further be divided into:
� To explore the effect of LLDPE contents on the mechanical, thermal and
morphological properties of PLA/LLDPE blends.
� To investigate the effect of MMT concentration on the mechanical, thermal and
morphological properties of PLA/MMT and PLA/LLDPE/MMT
nanocomposites.
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� To evaluate the water absorption behavior and hygrothermal aging of PLA/MMT
and PLA/LLDPE/MMT nanocomposites.
1.3 Scope of Research
In order to achieve the objectives of the research the following activities were
carried out:
1. Sample preparation
In this research project, sample preparation and blending was performed via melt
intercalation method. This involves:
i) Blending of LLDPE with PLA to produce PLA/LLDPE blends with twin screw
extruder.
ii) Blending of MMT with PLA to produce PLA/MMT nanocomposites with twin
screw extruder.
iii) Blending of LLDPE and MMT with PLA to produce PLA/LLDPE/MMT
nanocomposites with twin screw extruder.
iv) The blends and nanocomposites fabricated into test specimens via injection
molding for analysis.
2. Physical and mechanical analysis
(i) Water absorption
(ii) Hygrothermal aging
(iii)Tensile test
(iv) Flexural test
(v) Izod impact test
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3. Characterization and morphological study
(i) X-ray Diffraction (XRD)
(ii) Scanning electron microscopy (SEM)
(iii) Transmission electron microscopy (TEM)
4. Thermal properties analysis
(i) Differential scanning calorimeter (DSC)
(ii) Themogravimetric analysis (TGA)
(iii) Dynamic mechanical analysis (DMA)
1.4 Importance of Research
The research expects to develop a toughened nanocomposite from a renewable
resource with balanced mechanical properties. The success of this project will widen the
applications of PLA such as furniture, automobile parts and enables it to be considered
as a possible substitute for conventional petroleum based plastics.
